1. Field of the Invention
The present invention relates to an image pickup device, and more particularly to driving a solid state image pickup device made of photoelectric conversion elements such as semiconductor photodiodes, and charge-coupled devices.
2. Description of the Related Art
CCD transfer type solid state image pickup devices are known which are used with electronic cameras, copiers, and other video apparatus. A number of photodiodes are disposed in the vertical and horizontal directions to form a pixel matrix.
A vertical charge transfer path (VCCD) is formed adjacent to each photodiode column, and a horizontal charge transfer path (HCCD) is formed near the ends of respective VCCDs.
Reducing the size of a solid state image pickup device is strongly requested nowadays. The number of pixels of a solid state image pickup device in the vertical direction is stipulated by the specification of NTSC, PAL or the like. The number of photodiodes of an image pickup device is therefore the same for all chip sizes regardless of how much they are reduced, such as from 1 inch to 2/3 inch, 1/2 inch, and to 1/3 inch.
Three or more transfer electrodes per one photodiode are required in order to read electric charges of all photodiodes separately at the same time. There comes a limit of fine patterning as the chip size is reduced, and so it becomes impossible to form three or more electrodes per one photodiode.
According to the specification of NTSC, PAL, or the like, interlaced image signals are used and one frame is formed by scanning every other line twice. In this case, VCCDs having two transfer electrodes per one photodiode row can be used.
When light of an electronic flash lamp is used, the exposure start time is the same for all photodiodes. If the exposure end time is different, a total exposure time period becomes different. From the standpoint of the resolution of a moving image, a different exposure time period is not desirable. It is also desirable to have as many pixels as possible, in order to obtain a very fine image. Therefore, for taking a very fine still image under an electronic flash lamp light, it is desirable to read electric charges of all photodiodes at the same time.
As a method of reading all electric charges of an image pickup device of this type at the same time, an accordion transfer method has been proposed (PHILIPS TECHNICAL REVIEW, Vol. 43, No. 1/2, 1986, A. J. P. Theuwissen and C. H. L. Weijtens).
FIGS. 8A and 8B illustrate the operation of the accordion transfer method. FIG. 8A is a conceptual diagram showing how the potentials under electrodes along the transfer path change with time. FIG. 8B is a conceptual plan diagram showing how electric charges are transferred by using the accordion transfer method.
Throughout this specification, the term "potential" is intended to mean a potential energy, the lower potential having a stable state irrespective of the polarity of electric charges.
Referring to FIG. 8A, electrodes along the transfer path include odd numbered electrodes Od and even numbered electrodes Ev. A cell of the charge transfer path is formed under each electrode. The potentials under the odd numbered electrodes are lowered first to form potential wells in which electric charges qa, qb, and qc are stored. If the potential barriers between potential wells are lowered in this state, the electric charges will be mixed.
In order to avoid such a charge mixture, the potential under the rightmost even electrode is lowered first to extend the potential well in length by an amount corresponding to two electrodes. Therefore, the electric charge qa distributively propagates to the right by a one-electrode length. Next, the potential at the left portion of the potential well storing the electric charge qa is raised, and at the same time the potential at the right portion of the potential barrier is lowered. As a result, the electric charge qa is transferred to the right by a one-electrode length while being distributed over the two-electrode length.
At this time, a potential barrier corresponding to a two-electrode length is formed between the electric charges qa and qb. To transfer the electric charge qa farther to the right, an operation of raising the potential at the left portion of the well and at the same time lowering the potential at the right portion is repetitively carried out.
After the potential barrier corresponding to the two-electrode length is formed between the electric charges qa and qb, the potential of this barrier on the right side of the electric charge qb is lowered. As a result, the electric charge qb distributively propagates to the right by a two-electrode length. At this time, there is a potential barrier corresponding to at least one-electrode length, or two-electrode length in an ordinary case, between the electric charges qa and qb, preventing a charge mixture. In the manner described above, electric charges stored in wells at every other electrode are extended by a two-electrode length to transfer the charges.
FIG. 8B conceptually shows the distribution of electric charges transferred in the above manner. The abscissa represents time, and the ordinate represents electrodes along the transfer path. Under the state shown at the leftmost, electric charges qa, qb, qc, and qd are stored under every other electrode at the upper half of the transfer path. These electric charges are sequentially transferred downward, starting from the electric charge at the lowest position, while forming a potential well corresponding to a two-electrode length and a potential barrier corresponding to a two-electrode length.
Namely, during the charge transfer, an electric charge is distributively propagated by a two-electrode length and a potential barrier corresponding to a two-electrode length is formed between electric charges. It is therefore possible to transfer electric charges stored under every other electrode while preventing a charge mixture. Under the charge transfer completion state shown at the rightmost, the electric charges qa, qb, qc and qd distribute at every other electrode and take the original distribution pattern.
The manner of forming potential wells and barriers during the charge transfer is analogous to gradually opening and then closing the bellows of a musical instrument, like an accordion. This is the reason why this charge transfer method is called an accordion transfer method.
As shown in the potential diagram of FIG. 8A, the accordion transfer method uses four-phase drive signals.
The present application has proposed a charge transfer method similar to the above-described accordion transfer method. The proposed method is directed to a solid state image pickup device having a photodiode matrix, vertical charge transfer paths and horizontal charge transfer paths. According to the proposed method, not photodiodes but CCDs are used for the transfer path, and charges are transferred in response to four-phase drive signals like those for an interline type CCD. However, only one signal per two photodiode rows is allowed to be transferred.
FIGS. 9A and 9B show an FIT pseudo frame electronic shutter proposed by the present applicant. FIG. 9A is a schematic plane view showing the structure of the shutter, and FIG. 9B is a conceptual diagram showing the operation of the shutter.
Referring to FIG. 9A, a number of photodiodes P are disposed in a matrix shape, for example, by doping n-type impurities in a p-type silicon substrate. A plurality of charge transfer paths L made of CCDs are formed near respective columns of the photodiodes.
Transfer gates G are formed between the photodiodes P and the charge transfer paths L. Two electrodes per each photodiode row are formed on the charge transfer paths L.
Each charge transfer path L has a light receiving section R and a charge accumulating section S extending from the photodiode area to the area where the photodiodes are not formed. An HCCD is connected to the ends of the charge accumulation sections of the charge transfer paths L. An output from HCCD is read via a charge detecting amplifier.
The photodiodes P distributed in the matrix shape include odd numbered photodiodes PA and even numbered photodiodes PB, the former forming an A field and the latter forming a B field. The two fields A and B form one frame of an image.
If electric charges are read from all photodiodes at the same time and transferred, a charge mixture occurs because the charge transfer path has only two electrodes per one photodiode row.
The following operation is performed in order to read electric charges from all photodiodes without a charge mixture.
FIG. 9B shows the outline of reading electric charges from the photodiodes shown in FIG. 9A.
Electric charges stored in the odd numbered photodiodes PA are first read and stored in the charge transfer paths L at the light receiving section L (R). In this case, one electric charge signal per four electrodes is read and stored in the charge transfer path L.
Next, the electric charges read and stored in the charge transfer paths L (R) at the light receiving section R are transferred to the charge transfer paths L (S) at the charge accumulating section S. This charge transfer may be performed by using four-phase drive signals, without a charge mixture.
After the electric charges stored in the odd numbered photodiodes are transferred to the charge transfer paths L (S) at the charge accumulating section, electric charges stored in the even numbered photodiodes PB are read and stored in the charge transfer paths L (S) at the light receiving section. In this manner, the electric charges of the a field are stored in the charge transfer paths L at the charge accumulating section, and the electric charges of the B field are stored in the charge transfer paths L at the light receiving section.
Next, while holding in position the electric charges in the charge transfer paths L (R) at the light receiving section, the electric charges in the charge transfer paths L (S) at the charge accumulating section are sequentially transferred one row after another to the HCCD. The electric charges in the HCCD are transferred in the horizontal direction and picked up from the charge detecting amplifier.
After all the electric charge signals of the A field stored at the charge accumulating section have been read, the electric charge signals stored in the charge transfer paths L (R) at the light receiving section are transferred downward and sent one row after another to the HCCD. The electric charges in the HCCD are transferred in the horizontal direction and picked up from the charge detecting amplifier.
In the above manner, electric charge signals stored in all the photodiodes
and PB can be read. With this method, however, the image taking times required for the A and B fields are a little different, and the images are picked up at a little different times. Electric charges in the electric charge transfer paths L are transferred by four-phase drive signals like those for an interline type CCD.
In the accordion or domino transfer method, electric charges are transferred while being extended along the charge transfer paths. Accordingly, the electric charges stored in the charge transfer paths at its upper section are required to be held in position for a longer time period than the electric charges stored in the charge transfer paths at its lower section.